Method of forming tungsten film

ABSTRACT

A method of forming a tungsten film on a surface of an object to be processed in a vessel capable of being vacuumized, includes the steps of forming a tungsten film by alternately repeating a reduction gas supplying process for supplying a reduction gas and a tungsten gas supplying process for supplying a tungsten-containing gas with an intervening purge process therebetween for supplying an inert gas while vacuumizing the vessel. A reduction gas supplying period of a reduction gas supplying process among the repeated reduction gas supplying processes is set to be longer than that of the remaining reduction gas supplying processes.

FIELD OF THE INVENTION

The present invention relates to a method of forming a tungsten film ona surface of an object to be processed, e.g., a semiconductor wafer orthe like.

BACKGROUND OF THE INVENTION

Generally, in order to form a wiring pattern on a surface of asemiconductor wafer serving as an object to be processed or buryrecesses between wiring or contact holes during a semiconductorintegrated circuit manufacturing process, a metal or a metalic compoundsuch as W(tungsten), WSi(tungsten silicide), Ti(titanium), TiN(titaniumnitride), TiSi(titanium silicide), Cu(copper) or Ta₂O₅(tantalum oxide)is deposited to form a thin film. In such a case, a tungsten film iswidely uses since it has a small resistivity and requires a low filmadhesion temperature. In order to form such a tungsten film, tungstenhexafluoride (WF₆) is used as a source gas and is reduced by hydrogen,silane, dichlorosilane or the like, thereby depositing the tungstenfilm.

In case of forming such a tungsten film, a Ti film, a TiN film, or astack of both of those films is thinly and uniformly formed first on awafer surface as a barrier layer serving as an under film for thepurpose of improving the adhesivity and then suppressing a reaction withan underlying silicon layer and the tungsten film is deposited thereon.

When burying or remedying the recesses or the like, hydrogen gas havinga weaker reducing power than silane is mainly used to enhance theburying characteristics. At this time, the barrier layer may be attackedby unreacted WFs to react with fluorine and then be expanded in volume,thereby generating an upwardly protruded volcano or a void in a buriedhole.

The above phenomenon will be explained with reference to FIG. 13. FIG.13 is a cross sectional view of a buried hole having a volcano and avoid. A buried hole 2 such as a contact hole or the like is formed on asurface of a semiconductor wafer W. A barrier layer 4 formed of, e.g., aTi/TiN film, is formed in advance on the surface including an innersurface of the buried hole 2. In case of performing a burying process onthe structure described above by depositing a tungsten film 6 by way ofsimultaneously supplying WF₆ gas and H₂ gas, fluorine of the WF₆ gas isdiffused into the barrier layer 4. Especially, the fluorine reacts withTi in the barrier layer 4 on the inner surface, leading to a protrudeddeposition of the tungsten film 6 starting at the buried hole 2. As aresult, a volcano 8 can be generated at an end portion of the protrusionby a stress of the tungsten film 6 or a void 10 having a cavity shapecan also be generated inside the buried hole 2.

In order to prevent the generation of the volcano 8 or the like, silanehaving a stronger reducing power than that of the hydrogen gas was alsoused for forming a nucleation layer of the tungsten film 6 with a smallthickness of, e.g., about 300 to 500 Å. Thereafter, deposition of a maintungsten film was carried out starting at the nucleation layer by usingH₂ gas and WF₆ gas. In this case, however, the nucleation layer may notbe Uniformly formed due to, e.g., a contamination of a surface of thebarrier layer 4 serving as a base film.

Thus, prior to the formation of the nucleation layer, only silane isprovided for a certain time period to allow reaction intermediates ofthe silane (SiHx:x<4) to be absorbed on a wafer surface at a lowtemperature, e.g., 400° C. at which a part of the silane can bedecomposed. Then, the nucleation layer is grown starting at thatportion. FIGS. 14A to 14F illustrate the processes for charging a buriedhole with tungsten by using the above-described method.

As shown in FIG. 14A, an initiation process of adhering reactionintermediates 12, i.e., SiHx discussed above, on a surface of a wafer Wis performed on the wafer W having a barrier layer 4 formed on itsentire surface including an inner surface of a buried hole 2 (FIGS. 14Aand 14B). Next, as described above, by simultaneously supplying the WF₆gas and the SiH₄ gas for a certain time period as illustrated in FIG.14C, a tungsten film is deposited starting at the reaction intermediates12, thereby forming a nucleation layer 14 (FIG. 14D).

Subsequently, by simultaneously supplying the WF₆ gas and the H₂ gas asshown in FIG. 14E, a main tungsten film 16 is deposited so as to fillthe buried hole as illustrated in FIG. 14F.

In case of forming the barrier layer 4 formed on the wafer surface, anorganic compound source of Ti is normally used in order to increase astep coverage. However, a carbon component in the organic compoundsource is included in the barrier layer 4 and, thus, adhesion of thereaction intermediates to the barrier layer 4 becomes irregular despitethe initiation process due to the exposure of the carbon component on asurface of the barrier layer 4. Consequently, the nucleation layer 14 isirregularly formed thereon, and the step coverage thereof is alsodeteriorated, resulting in deteriorated burying characteristics of amain tungsten film, causing voids, volcanoes or the like.

Such a problem does not occur in a case where the ratio between thethickness of the nucleation layer 14 and that of the entire tungstenfilm including the main tungsten layer 16 is not so high. On the otherhand, in case the ration becomes non-negligibly high because of scalingdown, voids of a non-negligible size can be generated due to adeterioration of the step coverage of the nucleation layer 14.

The above problem becomes aggravated as a serious issue especially whenan inner diameter of a buried hole becomes smaller than or same to,e.g., 0.2 μm as a result of the manufacturing requirement for a furtherscaling down and a thinner film.

SUMMARY OF THE INVENTION

It is, therefore, a primary object of the present invention to provide amethod of forming a tungsten film of satisfactory buryingcharacteristics by preventing the generation of voids and volcanoes aslarge as adversely affecting the characteristics, even in a buried holehaving a small diameter.

In accordance with a preferred embodiment of the present invention,there is provided a method of forming a tungsten film on a surface of anobject to be processed in a vacuum vessel, the method including the stepof: forming an initial tungsten film by alternately repeating areduction gas supplying process for supplying a reduction gas and atungsten gas supplying process for supplying a tungsten-containing gaswith an intervening purge process therebetween for supplying an inertgas while vacumizing the vessel.

Accordingly, an initial tungsten film having a uniform thickness can beformed as a nucleation layer. Therefore, when a main tungsten film issubsequently deposited thereon, it is possible to restrict thegeneration of voids or volcanoes in sizes capable of adversely affectingthe burying characteristics, e.g., even in a buried hole having a smalldiameter.

In accordance with another preferred embodiment of the presentinvention, there is provided a method of forming a tungsten film on asurface of an object to be processed in a processing vessel capable ofbeing vacuumized, the method including the step of: forming an initialtungsten film by, in repetition, supplying a reduction gas and atungsten-containing gas with an intervening purge process therebetweenfor supplying an inert gas while vacuumizing the vessel, wherein thetotal pressure of the reduction gas, the tungsten-containing gas and theinert gas is controlled to be constant.

In accordance with the present invention, by controlling the totalpressure of the reduction gas, the tungsten-containing gas and the inertgas throughout the step of forming the initial tungsten film, thetemperature of a wafer (an object to be processed) and the amount of gasto be absorbed can be maintained regularly.

In the method of forming the tungsten film in accordance with thepresent invention, a parameter, which is obtained by multiplying apartial pressure of the reduction gas by the supplying time thereof inan initial reduction gas supplying process among the repeated reductiongas supplying processes, may be set to be greater than that in thesubsequent reduction gas supplying processes.

In accordance with the present invention, by performing a substantiallysame function as the initiation process of the conventional method, areaction intermediate may be adhered to a surface of an object to beprocessed so that the surface can be activated.

In accordance with still another preferred embodiment of the presentinvention, an initial tungsten film is formed by repeatedly supplying areduction gas and a tungsten-containing gas with an intervening purgeprocess for supplying an inert gas, while setting the supplying time ofa reduction gas in an initial reduction gas supplying process among therepeated reduction gas supplying processes to be longer than that of thesubsequent reduction gas supplying processes and controlling the totalpressure of the reduction gas, the tungsten-containing gas, and theinert gas to be constant throughout the step of forming the initialtungsten film.

In accordance with the present invention, the initial reduction gassupplying process functions in a same way as in the initiation processof the conventional method, and a temperature of a wafer (an object tobe processed) and the amount of gas to be absorbed thereon can bemaintained regularly by controlling the total pressure of the reductiongas, the tungsten-containing gas and the inert gas.

In accordance with the method of the present invention, after theinitial tungsten film is formed, a main tungsten film is formed bysimultaneously supplying the tungsten-containing gas and the reductiongas.

In accordance with the present invention, it is possible to prevent thegeneration of voids or volcanoes having sizes capable of adverselyaffecting the burying characteristics even in a buried hole having asmall diameter, thereby providing satisfactory burying characteristics.

In accordance with the method of the present invention, a process forforming a passivation tungsten film is performed between the initialtungsten film forming process and the main tungsten film forming processby simultaneously supplying the tungsten-containing gas and thereduction gas while setting the flow ratio of the tungsten-containinggas to be smaller than that supplied in the main tungsten film formingprocess.

In case the thickness of an initial tungsten film is thin, when the maintungsten film is formed, volcanoes may be generated by an attack of theWF₆ gas. However, in accordance with the present invention, by forming apassivation tungsten film functioning as a so-called passivation film,the initial tungsten film can be protected, thereby further improvingthe burying characteristics.

In accordance with the method of the present invention, the initialtungsten film forming process and the passivation tungsten film formingprocess are carried out in a substantially equal condition with respectto the process pressure and/or the process temperature.

In accordance with the method of the present invention, the maintungsten film forming process has at least either the process pressureor the process temperature substantially higher in comparison with thepassivation tungsten film forming process.

In the method of the present invention, the tungsten-containing gas maybe WF₆ gas or an organic tungsten source gas.

Further, in the method of the present invention, the reduction gas maybe selected from the group consisting of H₂, silane (SiH₄), disilane(Si₂H₆), dichlororosilane (SiH₂Cl₂), diboran (B₂H₆) and phospine (PH₃)

Further, in the method of the present invention, the tungsten-containinggas may preferably be WF₆ gas, and the reduction gas may preferably beSiH₄ gas in the initial tungsten film forming process and H₂ gas in thepassivation tungsten film forming process.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodiments,given in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross sectional view of a heat treatment device forperforming a method of forming a tungsten film in accordance with thepresent invention;

FIG. 2A shows a gas supply pattern wherein a purge process is intervenedbetween a reduction gas supplying process and a tungsten-containing gassupplying process;

FIG. 2B illustrates a gas supply pattern obtained by increasing aparameter of an initial reduction gas supplying process in the gassupply pattern of FIG. 2A, the parameter being (the partial pressure ofa reduction gas) (the supplying time) period;

FIG. 2C describes a gas supply pattern wherein a passivation tungstenfilm forming process is intervened between an initial tungsten filmforming process and a main tungsten film forming process in the gassupply pattern of FIG. 2B:

FIG. 2D depicts a gas supply pattern wherein a passivation tungsten filmforming process is intervened between an initial tungsten film formingprocess and a main tungsten film forming process in the gas supplypattern of FIG. 2A;

FIG. 3A presents a distribution of a partial pressure of silane (SiH₄)in a diffusion processing chamber in case a diffusion plate is installedinside a shower head;

FIG. 3B represents a distribution of a partial pressure of silane (SiH₄)in a diffusion processing chamber in case a diffusion plate is notinstalled inside a shower head;

FIG. 4 provides an enlarged cross sectional view of an exemplarytungsten film deposited on a surface of a semiconductor wafer;

FIG. 5 offers an enlarged cross sectional view of another exemplarytungsten film deposited on a semiconductor wafer surface;

FIG. 6 is a graph showing a relationship between a parameter (Torr·sec)of silane gas and the thickness of a film formed during one cycle;

FIG. 7 provides a graph illustrating a relationship between a parameter(Torr·sec) of WF₆ and the thickness of a film formed during one cycle;

FIG. 8 describes a graph depicting a temperature dependency of thethickness of a film formed during one cycle of a gas supply;

FIG. 9 illustrates a graph showing a relationship between a parameter(Torr·sec) of WF₆ and the number of volcanoes generated in one cell;

FIG. 10A presents a picture substituting a diagram showing a crosssectional view of a buried hole buried by using a conventional method;

FIG. 10B represents a picture substituting a diagram showing a crosssectional view of a buried hole buried by using a method of the presentinvention;

FIG. 11 shows a graph depicting a temperature dependency of aresistivity of a tungsten film;

FIG. 12 provides a graph illustrating a fluorine F concentration (theamount of diffusion) profile of a wafer surface;

FIG. 13 describes a cross sectional view of a buried hole with a volcanoand a void generated;

FIG. 14A provides an embodiment of a process for filling with tungsten aburied hole formed on the wafer w having a barrier layer formed on itsentire surface including an inner surface of the buried hole;

FIG. 14B offers an embodiment of a process for filling with tungsten theburied hole formed on the wafer W to which reaction intermediates, i.e.,SiHx, are adhered;

FIG. 14C presents an embodiment of a process for filling with tungstenthe buried hole formed on the wafer W wherein a nucleation layer isdeposited starting from the reaction intermediates;

FIG. 14D represents an embodiment of a process for filling with tungstenthe buried hole formed on the wafer W having the nucleation layer formedthereon;

FIG. 14E shows an embodiment of a process for filling with tungsten theburied hole on the wafer W having the nucleation layer formed thereon bysupplying a reduction gas and a tungsten-containing gas; and

FIG. 14F sets forth an embodiment of a process for filling with tungstenthe buried hole formed on the wafer W having a main tungsten film formedthereon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of a method of forming a tungstenfilm in accordance with the present invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a cross sectional view of a heat treatment device forperforming a method of forming a tungsten film in accordance with thepresent invention. FIGS. 2A to 2D provide supply patterns of each gas.FIG. 4 illustrates an enlarged cross sectional view of an exemplarytungsten film deposited on a semiconductor wafer surface. FIG. 5presents an enlarged cross sectional view of another exemplary tungstenfilm deposited on a semiconductor wafer surface.

First of all, an explanation of a heat treatment device for performingthe method of the present invention will be provided. The heat treatmentdevice 20 has a processing vessel 22 having an approximately cylindricalcross-section, for example, which is made of aluminum. Installed on aceiling portion of the processing vessel 22 is a shower head 24 forselectively introducing a flow rate controlled processing gas, e.g.,various film forming gases, carrier gases and the like, via a sealingmember 26 such as an O-ring and the like. Further, installed at a bottomportion of the shower head 24 is a plurality of gas jetting holes 28 forspraying a film forming gas toward a processing space S. Further, in theshower head 24, there may be installed one or more of diffusion plates27 each having a plurality of diffusion holes 25 so as to facilitate adiffusion of a gas introduced therein.

Meanwhile, in the processing vessel 22, a mounting table 34 for loadingthereon a semiconductor wafer W as an object to be processed isinstalled on a cylindrical reflector 30 built on a bottom portion of theprocessing vessel via, e.g., L-shaped three support members 32 (only twothereof are shown in FIG. 1).

Installed below the mounting table 34 are a plurality of, e.g., threeL-shaped lifer pins 36 (only two thereof are illustrated in thisexample) rising upward. Base portions of the lifter pins 36 are commonlyconnected to a ring member 38 through vertical through holes (now shown)formed at the reflector 30. The ring member 38 is moved vertically by adrive up rod 40 passing through the bottom portion of the processingvessel, such that the lifter pins 36 can pass through lifter pin holes42 formed through the mounting table 34, to thereby lift the wafer W.

An expansible/contractible bellows 44 is installed around the throughhole for the drive up rod 40 formed at the bottom portion of theprocessing vessel 22, so that an inner space of the processing vessel 22can be kept air-tight. Further, a bottom portion of the drive up rod 40is connected to an actuator 46.

At a peripheral portion of the bottom portion of the processing vessel22, an exhaust port 48 is formed to be connected to an exhaustpassageway 50 being connected to a vacuum pump (not shown), so that itis possible to exhaust the inner space of the processing vessel 22 downto a predetermined vacuum level. Further, installed at a sidewall of theprocessing vessel 22 is a gate valve 52 to be opened and closed when awafer is loaded thereinto or unloaded therefrom.

Although not illustrated, a vacuum gage (capamanometer) for measuringpressure is installed in the processing vessel 22, and an automaticpressure control valve for controlling pressure in the processing vessel22 is installed at the exhaust passageway 50.

Further, air-tightly installed at the bottom portion of the processingvessel below the mounting table 34 is a transparent window 54 made of aheat ray transmission material, e.g., quartz or the like, via a sealingmember 56 such as an O-ring or the like. Under the transparent window54, a box-shaped heating chamber 58 is installed to surround thetransparent window 54. The heating chamber 58 has therein a heatingdevice, e.g., a plurality of heating lamps 60 disposed on a rotatabletable 62 also serving as a reflector. The rotatable table 62 is revolvedby a rotating motor 64 installed at a bottom portion of the heatingchamber 58 via a rotation axis. Heat rays emitted from the heating lamps60 transmit the transparent window 54 and then are irradiated on abottom portion of the thin mounting table 34, thereby heating the bottomportion thereof and indirectly heating the wafer W on the mounting table34. By using the heating lamps 60 in this way, it is possible to greatlyincrease a heating rate.

Hereinafter, the method of the present invention, which is implementedby using the above-described device, will be described, First, the gatevalve 52 installed at the sidewall of the processing vessel 22 isopened. Then, a wafer W is loaded into the processing vessel 22 by atransfer arm (not shown) and by lifting the lifter pins 36, the wafer Wis transferred to the lifer pins 36. Next, the lifter pins 36 descend bylowering the drive up rod 40, so that the wafer W can be loaded on themounting table 34. Provided as an under film on a surface of the wafer Wincluding an inner surface of the buried hole 2 is a barrier layer 4,e.g., a Ti/TiN film formed during a previous process (see FIG. 14A).

Thereafter, predetermined amounts of film forming gas, carrier gas andthe like serving as a processing gas is supplied from a processing gassource (not shown) to the shower head 24 and then substantiallyuniformly supplied into the processing vessel 22 through the gas jettingholes 28 according to a gas supply pattern, which will be describedlater. At the same time, an inner atmosphere of the processing vessel 22is exhausted through the exhaust port 48, such that the interior of theprocessing vessel 22 can be kept at a predetermined pressure level.Further, each of the heating lamps 60 of the heating device disposedunder the mounting table 34 is rotated so as to emit a heat energy.

The emitted heat source transmits the transparent window 54 and then isirradiated on a backside of the mounting table 34, thereby heating thebackside thereof. The mounting table 34, as described above, has a verylow thickness of, e.g., 1 mm and, therefore, is heated rapidly. Thus,the wafer loaded on the mountain table 34 can also be quickly heated toa predetermined temperature. The supplied film forming gas is subjectedto a chemical reaction, resulting in a deposition of a tungsten thinfilm on an entire surface of the wafer W.

Hereinafter, supply patterns of each gas will be described in detailwith reference to FIGS. 2A to 2D.

Referring to Figs, 2A to 2D, there are illustrated three types of gassupply patterns. In each of the patterns, a carrier gas, e.g., Ar gas orN₂ gas, is continuously supplied at a constant flow rate or a variableflow rate. Further, an inner space of the processing vessel 22 iscontinuously exhausted during a series of processes.

Herein, Was gas is used as a tungsten-containing gas, and H₂ or SiH₄ gashaving a stronger reducing power than that of the H₂ gas is used as areduction gas.

A gas supply pattern illustrated in FIG. 2A forms an initial tungstenfilm 76 (see FIG. 4) by alternately repeating a reduction gas supplyingprocess 70 for supplying a reduction gas, i.e., SiH₄ gas and atungsten-containing gas supplying process 72 for supplying a tungstengas, i.e., WF₆ gas, with a purge process 74, intervened therebetween,for supplying a carrier gas as an inert gas while exhausting in vacuum,intervened therebetween. In other words, an initial tungsten filmforming process is carried out by alternately repeating the SiH₄ gassupplying process and the WF₆ gas supplying process while interveningthe purge process 74 therebetween. Such an initial tungsten film formingprocess is completed by performing the reduction gas supplying process70. Such a sequence of the processes also apply to cases of FIGS. 2B to2D.

After the initial tungsten film 76 is formed, a H₂ gas instead of theSiH₄ gas is used as a reduction gas. Then, by simultaneously supplyingthe H₂ gas and WF₆ gas that is a tungsten-containing gas together withan inert gas, e.g., Ar gas or N₂ gas, a main tungsten film formingprocess 80 for forming a main tungsten film 78 is performed, therebycompletely filling the buried hole 2 with the main tungsten film 78.

Herein, if a period between a reduction gas supplying process 70 and itsnext reduction gas supplying process 70 is assumed to be one cycleduring the initial tungsten film forming process, three cycles areperformed in a case of FIG. 2A. However, the number of cycles is notlimited thereto.

A period T1 of each reduction gas supplying process 70 and a period T2of each tungsten-containing gas supplying process 72 are respectively 1to 30 seconds and, preferably, 3 to 10 seconds. Further, a period T3 ofthe purge process 74 is 0 to 30 seconds and, preferably, 0 to 10seconds. Furthermore, in the purge process 74, only a vacuumizingprocess may be performed.

Preferably, the total pressure of the reduction gas, thetungsten-containing gas and the inert gas is controlled to be constantthroughout the reduction gas supplying processes 70, thetungsten-containing gas supplying processes 72 and the purge processes74. By maintaining the constant total pressure of the gases, it ispossible to uniformly maintain a temperature of the wafer (an object tobe processed) and an absorbed amount of gases being absorbed. The totalpressure of the gases is controlled by measuring a pressure in theprocessing vessel 22 by employing the vacuum gauge installed at theprocessing vessel 22 and controlling the auto pressure control valveinstalled at the exhaust passageway 50.

In the ensuing discussion, a result of evaluation on the time period ofthe purge process 74 will be described in detail.

FIGS. 3A and 3B illustrate a distribution of a partial pressure ofsilane (SiH₄) in the processing vessel 22. FIG. 3A indicates a casewhere the diffusion plate 27 is installed in the shower head 24 whileFIG. 3B represents a case where the diffusion plate 27 is not installedin the shower head 24, wherein the X-axis presents a distance from thecenter of the wafer in a radius direction. Herein, a partial pressure ofthe SiH₄ remaining on the wafer W was measured when the purge process 74was performed for a few (0 to 3) seconds right after the SiH₄ supply wasstopped.

As can be clearly seen from FIGS. 3A and 3B, in case the diffusion plate27 is installed in the shower head 24 (FIG. 3A), the partial pressurewas lowered a little more quickly. That is, in case of FIG. 3A, byperforming the purge process 74 for about 1.5 seconds, the partialpressure of SiH₄ can be reduced to about 1×10⁻¹ Pa. Further, in case ofFIG. 3B, by performing the purge process 74 for about 3 seconds, thepartial pressure of SiH₄ can be decreased to about 1×10⁻¹ Pa.

Therefore, regardless of a structure of the shower head 24, if the purgeprocess 74 is performed for at least about 3 seconds, the partialpressure of the remaining silane can be reduced to about zero, so thatit is possible to ignore effects of a gas phase reaction.

Returning to FIG. 2A, partial pressure ratios of the SiH₄ gas and theWF₆ gas are reduced by relatively decreasing flow rates thereof.Further, a process temperature is set to be, e.g., 200 to 500° C., and,preferably, set to be low at 200 to 450° C. Further, the thickness of aninitial tungsten film formed during one cycle is 1 to 50 Å and,preferably, 3 to 20 Å.

A period of the main tungsten film forming process 80 depends on thethickness of a film to be formed. Herein, flow rates of both WF₆ gas andH₂ gas are increased. Further, a process pressure and a processtemperature are also slightly increased, to thereby set a film formingrate to be higher.

By such, the initial tungsten film 76 is substantially uniformlydeposited with high quality on the surface of the wafer W. Since theinitial tungsten film 76 functions as a nucleation layer 14 shown inFIG. 14C, it is possible to deposit thereon the main tungsten film 78having satisfactory burying characteristics.

In the gas supply pattern illustrated in FIG. 2B, a parameter(Torr·sec), which is obtained by multiplying a partial pressure (Torr)of the reduction gas by the supplying time (sec) thereof, of an initialreduction gas supplying process 70A is set to be greater than that ofthe remaining reduction gas supplying process 70 among the repeatingreduction gas supplying processes in the gas supply pattern shown inFIG., 2A. Herein, a period T4 of the initial reduction gas supplyingprocess 70A is set to be higher, e.g., 1 to 120 seconds, and,preferably, 30 to 90 second, without changing a flow rate of the SiH₄gas, to increase the parameter (Torr·sec).

In this way, by performing only an initial SiH₄ gas supplying processfor a long time, an initiation process is performed on the wafer Wsurface, so that the reaction intermediates, i.e., SiH_(X), can beadhered thereon as described with reference to FIG. 14B. Accordingly, itis possible to form a tungsten film 76 having a thickness of an improveduniformity.

Further, in the gas supply pattern shown in FIG. 2C, the passivationtungsten film forming process 84 for forming a passivation tungsten film82 (see FIG. 5) is performed right before the main tungsten film formingprocess 80 in the gas supply pattern illustrated in FIG. 2B. In thepassivation tungsten film forming process 84, gas species, i.e., WF₆ gasand H₂ gas, used in the main tungsten film forming process 80 are used.However, the flow ratio of a tungsten-containing gas is set to besmaller than that of S the main tungsten film forming process 80. Aperiod T5 of the passivation tungsten film forming process 84 is, e.g.,3 to 90 seconds and, preferably, 10 to 60 seconds.

In the gas supply pattern shown in FIG. 2D, the passivation tungstenfilm forming process 84 is successively performed right before the maintungsten film forming process 80 in the gas supply pattern illustratedin FIG. 2A.

Since the passivation tungsten film functions as a so-called passivationfilm, a damage of a Ti film due to a diffusion of F of WF₆ can beprevented in the main tungsten film forming process, thereby furtherimproving the burying characteristics.

Hereinafter, each process condition in the gas supply pattern of FIG. 2Cwill be described. In the initial reduction gas supplying process 70A, agas ratio of SiH₄ to a carrier gas is 90 sccm/8550 sccm, a processpressure is 80 Torr (10640 Pa) and a processing period T4 is 60 seconds.A maximum value of a process temperature is 200 to 500° C. and,preferably, 250 to 450° C., so as to avoid volcanoes or improve a stepcoverage.

Further, in this process, the parameter (Torr·sec), which is obtained bymultiplying the partial pressure of SiH₄ gas by a supply time thereof,is set to 10 to 300 Torr·sec and, preferably, 30 to 200 Torr·sec inorder to avoid volcanoes.

In the initial tungsten forming process, a gas ratio of SiH₄ to thecarrier gas in a second and its subsequent reduction gas supplyingprocesses 70 is 90 sccm/3900 sccm, a period T1 thereof is 5 seconds anda process pressure is 7.5 Torr (998 Pa). A process temperature is 200 to500° C. and, preferably, 250 to 450° C. At this time, the parameter is0.1 to 10 Torr·sec and, preferably, 0.2 to 5 Torr·sec for the purpose ofsaturating the film thickness.

Further, in the tungsten-containing gas forming process 72, a gas ratioof WF₆ to a carrier gas is 30 sccm/3900 sccm, a period T2 thereof is 5seconds and a process pressure is 7.5 Torr (998 Pa). A processtemperature is 200 to 500° C. and, preferably, 250 to 450° C. At thistime, the parameter (the partial pressure×the supplying time of the WF₆gas) is set to 0.01 to 0.6 Torr·sec and, preferably, 0.04 to 0.5Torr·sec in order to saturate the film thickness, thereby avoidingvolcanoes.

In the following, a detailed description of the reduction gas supplyingprocess 70 and the tungsten-containing gas supplying process 72 will begiven. FIG. 6 provides a graph showing a relationship between theparameter (Torr·sec) of silane at about 280° C. and the thickness of afilm formed during one cycle. In case the parameter is greater than orequal to 0.2, the film thickness is approximately saturated. On theother hand, in case the parameter is smaller than 0.2, the filmthickness depends on a value of the parameter. That is, in order to formthe initial tungsten film 76 having a predetermined thickness, theparameter is set to be 0.1 to 10 and, preferably, 0.2 to 5 to stabilize,e.g., in terms of its quality and uniformity, the thickness of the filmformed during one cycle. As a result, it is possible to saturate andstabilize, e.g., in terms of its quality and uniformity, the filmthickness under various process conditions.

FIG. 7 offers a graph illustrating a relationship between the parameter(Torr·sec) of WF₆ at about 280° C. and the thickness of a film formedduring one cycle, In case the parameter is greater than or equal to0.04, the film thickness is approximately saturated. On the other hand,if the parameter is smaller than 0.04, the film thickness depends on avalue of the parameter. That is, as described above, in order tostabilize, e.g., in terms of its quality and uniformity, the thicknessof the film formed during one cycle, the parameter is set to be 0.01 to10 and, preferably, 0.04 to 5.

FIG. 8 presents a graph showing a temperature independency of thethickness of a film formed during one cycle of a gas supply. In FIG. 8,there is illustrated the film thickness per one cycle when SiH₄ and WF₆are alternately supplied 90 times (90 cycles). In the graph, the X-axisindicates an actual wafer temperature.

As can be clearly seen from the graph, in case the wafer temperature islower than or equal to 100° C., the film is not deposited on the wafer.From 200 to 300° C., a film forming rate is gradually increased with theincrease of temperature. Then, at 300° C. and above, it is clear thatthe film forming rate is rapidly increased. Therefore, it is preferableto set the water temperature (being slightly lower than a processtemperature) to be greater than same to 100° C. in view of the filmthickness.

FIG. 9 depicts a graph illustrating a relationship between the parameter(Torr·sec) of WF₆ gas and the number of volcanoes generated in one cell.Herein, one cell indicates a group including therein about 50,000contact holes. According to the graph, in case the parameter is smallerthan or same to 0.5, volcanoes are not generated. However, if theparameter is greater than 0.5, the number of generated volcanoes isincreased substantially linearly. Under various process conditions, theparameter of the WF₆ gas is 0.01 to 0.6 and, preferably, 0.04 to 0.5.The thickness of the initial tungsten film 76, which may be affected byan inner diameter of the buried hole 2, is, e.g., 10 to 200 Å and,preferably, 20 to 150 Å.

Next, in the passivation tungsten film forming process 84, a gas ratioof WF₆ gas, H₂ gas and a carrier gas is deposit thereon the maintungsten film 78 having satisfactory burying characteristics.

In the gas supply pattern illustrated in FIG. 2B, a parameter(Torr·sec), which is obtained by multiplying a partial pressure (Torr)of the reduction gas by the supplying time (sec) thereof, of an initialreduction gas supplying process 70A is set to be greater than that ofthe remaining reduction gas supplying process 70 among the repeatingreduction gas supplying processes in the gas supply pattern shown inFIG. 2A. Herein, a period T4 of the initial reduction gas supplyingprocess 70A is set to be higher, e.g., 1 to 120 seconds, and,preferably, 30 to 90 second, without changing a flow rate of the SiH₄gas, to increase the parameter (Torr·sec).

In this way, by performing only an initial SiH₄ gas supplying processfor a long time, an initiation process is performed on the wafer Wsurface, so that the reaction intermediates, i.e., SiH_(x), can beadhered thereon as described with reference to FIG. 14B. Accordingly, itis possible to form a tungsten film 76 having a thickness of an improveduniformity.

Further, in the gas supply pattern shown in FIG. 2C, the passivationtungsten film forming process 84 for forming a passivation tungsten film82 (see FIG. 5) is performed right before the main tungsten film formingprocess 80 in the gas supply pattern illustrated in FIG. 2B. In thevolcanoes and obtain somewhat high step coverage and film forming rateat the same time. In this process, a process pressure is 40 Torr (5320Pa) and a process temperature is 300 to 500° C. and, preferably, 350 to450° C. In this case, the process pressure is set to be within a rangeof 20 to 200 Torr (2660 to 26600 Pa) in order to avoid the generation ofvolcanoes. Further, a maximum value of the process temperature is 300 to500° C. and, preferably, 350 to 450° C., thereby avoiding the generationof volcanoes and somewhat high step coverage and the film forming rate.

With respect to the partial pressure of the WF₆ gas, a minimum valuethereof is about 0.4 Torr (53 Pa) so as to increase the step coverage toa certain degree while a maximum value thereof is about 2.0 Torr (266Pa) in order to avoid the generation of volcanoes under the processpressure of smaller than or equal to 40 Torr. Moreover, a gas ratio ofWF₆ to H₂ is set to be 0.01 to 1 and, preferably, 0.1 to 0.5, to therebyincrease the step coverage to a certain degree and avoid the generationof volcanoes.

In addition, at least either the process pressure or the processtemperature of the main tungsten film forming process 80 is set to besubstantially higher, in comparison with the passivation tungsten filmforming process 84. In this way, the film forming rate of the maintungsten film forming process 80 is increased. Especially, it ispreferable to form a tungsten film during a transition between theprocesses, so that a speed of forming the tungsten film can beincreased, the process temperature being increased from 350° C. to 400°C. during the transition in the exemplary case described herein.

After the burial was actually performed by using the method of thepresent invention as shown in FIG. 2C, results illustrated in FIGS. 10Aand 10B were obtained.

FIGS. 10A and 10B respectively provide pictures of cross sectional viewsof buried holes that are buried by using a conventional method and themethod of the present invention. In case of the conventional methodshown in FIG. 10A, an unsatisfactory result was obtained due to a voidgenerated in the buried hole. On the other hand, in case of the methodof the present invention illustrated in FIG. 10B, no void was notgenerated in the buried hole, obtaining satisfactory buryingcharacteristics. In this case, an inner diameter of the buried hole was0.13 μm. Thus, in case of a minute hole greater than or equal to 0.13μm, the method of the present invention is effective. Besides,satisfactory effects were also obtained in case of a minute hole smallerthan or equal to 0.13 μm.

After forming a tungsten film having a thickness ranging from 100 Å to300 Å, a surface roughness thereof was examined by an electronmicroscope. In case the tungsten film is formed by employing aconventional CVD method, the surface roughness of the film was increasedas the film thickness was increased from 100 Å to 300 Å. However, incase of the method of the present invention, it has been proven that thesurface roughness of the, film is nearly stable regardless of the filmthickness and that a smooth surface of the tungsten film is provided.

Resistivity measurement was also carried out on the tungsten filmsdescribed above and the evaluation result thereof will be described withreference to FIG. 11.

FIG. 11 depicts a graph showing a temperature independency ofresistivities of tungsten films. In the graph, (a) indicates a tungstenfilm formed by employing the conventional CVD method (a processtemperature ≈400° C.); (b) represents a tungsten film formed at aprocess temperature of 280° C. by using the method of the presentinvention; and (c) depicts a tungsten film formed at the processtemperature of 380° C. by using the method of the present invention.

As can be clearly seen from the graph, the films (b) and (c) formed byusing the method of the present invention have an about twice to fourtimes greater resistivities than that of the film (a) formed by usingthe conventional CVD method. This is because the films (b) and (c) havea twice to tour times smaller microcrystalline size than that of thefilm (a). Further, between the films (b) and (c) formed by the method ofthe present invention, a film formed at a higher temperature has agreater resistivity. This is to be believed because the film formed atthe higher temperature contains Si of high concentration.

Furthermore, a result of examining a concentration of fluorine (F)diffused on a wafer surface will be described.

FIG. 12 illustrates a graph showing a profile of F concentration (anamount of diffusion) on the wafer surface. A wafer used herein has a TiNfilm, a Ti film, and an SiO₂ film that are sequentially formed under theW film (tungsten film).

As can be clearly seen from the graph, the F concentration of the Tifilm formed in accordance with the method of the present invention B is1×10¹⁷ atms/cc while the F concentration of the Ti film formed by theconventional CVD method A is 3×10¹⁷ atms/cc. In other words, the amountof F diffusion in the Ti film formed in accordance with the presentinvention B is suppressed at about one third of that formed in theconventional CVD method A, which represents higher barriercharacteristics.

Though hydrogen and silane were used as a reduction gas in the abovepreferred embodiment, disilane (Si₂H₆), dichororosilane (SiH₂Cl₂),diborane (B₂H₆), phosphine (PH₃) or a mixture thereof can also be used.In this case, it is preferable to use a gas having a stronger reducingpower in the initial tungsten film forming process than in the maintungsten film forming process 80.

Further, a same reduction gas may be used in the initial tungsten filmforming process, the passivation tungsten film forming process and themain tungsten film forming process.

Although SiH₄ was used in the initial tungsten film forming process, anH radical (an active species) generated by using plasma or ultravioletlight may be used.

As the tungsten-containing gas, WF₆ gas was used but an organic tungstensource gas may also be used.

In this preferred embodiment, a wafer was described as an object to beprocessed. However, the present invention is not limited thereto but maybe applied to an LCD substrate, a glass substrate and the like.

As described above, the method of forming a tungsten film in accordancewith the present invention can provide the following advantageousoperative effects.

In accordance with the present invention, an initial tungsten film canbe formed as a nucleation layer having high film thickness uniformity.Therefore, when a main tungsten film is subsequently deposited thereon,even if a diameter of a buried hole is small for example, it is possibleto avoid the generation of voids or volcanoes as large as those that canadversely affect the burying characteristics.

Further, since the present invention provides a substantially samefunction as an initiation process of the conventional method, reactionintermediates can be adhered to a surface of an object to be processed,so that the surface can be activated.

Furthermore, in accordance with the present invention, even in case asemiconductor device becomes miniaturized, e.g., if a diameter of aburied hole is smaller than or equal to 0.1 μm, it is possible to avoidthe generation of voids and volcanoes having sizes as large as thosewhich can adversely affect the burying characteristics.

Moreover, in accordance with the present invention, singe thepassivation tungsten film functions as a so-called passivation film, itis possible to further improve the burying characteristics.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A method of forming a tungsten film on a surface of an object to beprocessed in a vessel capable of being vacuumized, the methodcomprising: forming a tungsten film by alternately repeating a reductiongas supplying process for supplying a reduction gas to the vessel and atungsten gas supplying process for supplying a tungsten-containing gasto the vessel with an intervening purge process therebetween forsupplying an inert gas to the vessel while exhausting the vessel,wherein the tungsten film is formed by alternately repeating thereduction gas supplying process for supplying the reduction gas to thevessel and the tungsten gas supplying process for supplying thetungsten-containing gas to the vessel while controlling the totalpressure of the gases to be constant in the vessel throughout the stepof forming the tungsten film, and wherein a reduction gas-supplyingperiod of an initial reduction gas supplying process among the repeatedreduction gas supplying processes for supplying a reduction gas to thevessel is set to be longer than that of the remaining reduction gassupplying processes for supplying a reduction gas to the vessel.
 2. Themethod of claim 1, wherein a parameter, which is obtained by multiplyinga partial pressure of the reduction gas by the supplying period thereofin the initial reduction gas supplying process among the repeatedreduction gas supplying processes, is set to be greater than that in theremaining reduction gas supplying processes.
 3. The method of claim 1,wherein the tungsten-containing gas is WF₆ gas or an orgarnc tungstensource gas.
 4. The method of claim 1, wherein the reduction gas isselected from the group consisting of H₂ gas, silane (SiH₄), disilane(Si₂H₆), dichlororosilane (SiH₂Cl₂), diborane (B₂H₆) and phospine (PH₃).5. A method of forming a tungsten film on a surface of an object to beprocessed in a vacuum vessel, the method comprising the steps of:forming an initial tungsten film by alternately repeating a reductiongas supplying process for supplying a reduction gas to the vessel and atungsten gas supplying process for supplying a tungsten-containing gasto the vessel with an intervening purge process therebetween forsupplying an inert gas to the vessel while exhausting the vessel; and,forming a main tungsten film on the initial tungsten film bysimultaneously supplying the tungsten-containing gas and the reductiongas to the vessel after the initial tungsten film is formed, whereinduring the step of forming the initial tungsten film, the initialtungsten film is formed by alternately repeating the reduction gassupplying process for supplying the reduction gas to the vessel and thetungsten gas supplying process for supplying the tungsten-containing gasto the vessel while controlling the total pressure of the gases to beconstant throughout the step of forming the initial tungsten film, andwherein during the step of forming the initial tungsten film, areduction gas supplying period of an initial reduction gas supplyingprocess among the repeated reduction gas supplying processes forsupplying a reduction gas to the vessel is set to be longer than that ofthe remaining reduction gas supplying processes for supplying areduction gas to the vessel.
 6. The method of claim 5, wherein duringthe step of forming the initial tungsten film, a parameter, which isobtained by multiplying a partial pressure of the reduction gas by thesupplying period thereof in an initial reduction gas supplying processamong the repeated reduction gas supplying processes is set to begreater than that in the remaining reduction gas supplying processes. 7.The method of claim 5, wherein the tungsten-containing gas is WF₆ gas oran organic tungsten source gas.
 8. The method of claim 5, wherein thereduction gas is selected from the group consisting of H₂ gas, silane(SiH₄), disilane (Si₂H₆), dichlororosilane (SiH₂Cl₂), diborane (B₂H₆)and phospine (PH₃).
 9. A method of forming a tungsten film on a surfaceof an object to be processed in a vacuum vessel, the method comprisingthe steps of: forming an initial tungsten film by alternately repeatinga reduction gas supplying process for supplying a reduction gas to thevessel and a tungsten gas supplying process for supplying atungsten-containing gas to the vessel with an intervening purge processtherebetween for supplying an inert gas to the vessel while exhaustingthe vessel; forming a passivation tungsten film on the initial tungstenfilm by supplying the tungsten-containing gas and the reduction gas tothe vessel; and forming a main tungsten film on the passivation tungstenfilm by simultaneously supplying the tungsten-containing gas and thereduction gas to the vessel, wherein the passivation tungsten film isformed by simultaneously supplying the tungsten-containing gas and thereduction gas to the vessel while controlling a flow ratio of thetungsten-containing gas to be smaller than that in the main tungstenfilm forming step, and wherein during the step of forming the initialtungsten film, the initial tungsten film is formed by alternatelyrepeating the reduction gas supplying process for supplying thereduction gas to the vessel and the tungsten gas supplying process forsupplying the tungsten-containing gas to the vessel while controllingthe total pressure of the gases to be constant throughout the step offorming the initial tungsten film, and wherein during the step offorming the initial tungsten film, a reduction gas supplying period ofan initial reduction gas supplying process among the repeated reductiongas supplying processes for supplying a reduction gas to the vessel isset to be longer than that of the remaining reduction gas supplyingprocesses for supplying a reduction gas to the vessel.
 10. The method ofclaim 9, wherein during the step of forming the initial tungsten film, aparameter, which is obtained by multiplying a partial pressure of thereduction gas by the supplying period thereof, in the initial reductiongas supplying process among the repeated reduction gas supplyingprocesses is set to be greater than that in the remaining reduction gassupplying processes.
 11. The method of claim 9, wherein the initialtungsten film forming step and the passivation tungsten film formingstep have at least either a process pressure or a process temperaturesubstantially equal.
 12. The method of claim 9, wherein the maintungsten film forming step has at least either a process pressure or aprocess temperature substantially higher than the passivation tungstenfilm forming step.
 13. The method of claim 9, wherein thetungsten-containing gas is WF₆ gas or an organic tungsten source gas.14. The method of claim 9, wherein the reduction gas is selected fromthe group consisting of H₂ gas, silane (SiH₄), disilane (Si₂H₆),dichlororosilane (SiH₂Cl₂), diborane (B₂H₆) and phospine (PH₃).
 15. Themethod of claim 9, wherein, in the initial tungsten film forming step,the tungsten-containing gas is WF₆ gas, and the reduction gas includesSiH₄ gas, and wherein, in the passivation tungsten film forming step,the tungsten-containing gas is WF₆ gas and the reduction gas includes H₂gas.
 16. A method of forming a tungsten film on a surface of an objectto be processed in a vacuum vessel, the method comprising the steps of:forming a tungsten film by alternately repeating a reduction gassupplying process for supplying a reduction gas to the vessel and atungsten gas supplying process for supplying a tungsten-containing gasto the vessel with an intervening purge process therebetween forsupplying an inert gas to the vessel while exhausting the vessel; andwherein forming the tungsten film is completed by performing thereduction gas supplying process, and wherein during the step of formingthe tungsten film, the tungsten film is formed by alternately repeatingthe reduction gas supplying process for supplying a reduction gas to thevessel and the tungsten gas supplying process for supplying a tungstencontaining gas to the vessel while controlling the total pressure of thegases to be constant throughout the step of forming the tungsten film,and wherein during the step of forming the tungsten film, a reductiongas supplying period of a reduction gas supplying process among therepeated reduction gas supplying processes for supplying a reduction gasto the vessel is set to be longer than that of the remaining reductiongas supplying processes for supplying a reduction gas to the vessel. 17.The method of claim 16, wherein during the step of forming the tungstenfilm, a parameter, which is obtained by multiplying a partial pressureof the reduction gas by the supplying period thereof in a reduction gassupplying process among the repeated reduction gas supplying processesis set to be greater than that in the remaining reduction gas supplyingprocesses.
 18. The method of claim 16, wherein the tungsten-containinggas is WF₆ gas or an organic tungsten source gas.
 19. The method ofclaim 16, wherein the reduction gas is selected from the groupconsisting of H₂ gas, silane (SiH₄), disilane (Si₂H₆), dichlororosilane(SiH₂Cl₂), diborane (B₂H₆) and phospine (PH₃).
 20. The method of claim1, wherein a TiN film is formed on the surface of the object, andwherein the tungsten film is formed on the TiN film.
 21. The method ofclaim 5, wherein a TiN film is formed on the surface of the object, andwherein the initial tungsten film is formed on the TiN film.
 22. Themethod of claim 9, wherein a TiN film is formed on the surface of theobject, and wherein the initial tungsten film is formed on the TiN film.23. The method of claim 2, wherein the parameter is about 10 to 300Torr·sec.
 24. The method of claim 6, wherein the parameter is about 10to 300 Torr·sec.
 25. The method of claim 10, wherein the parameter isabout 10 to 300 Torr·sec.
 26. The method of claim 1, wherein the methodis performed in a temperature ranging from about 200 to 500° C.
 27. Themethod of claim 5, wherein the method is performed in a temperatureranging from about 200 to 500° C.
 28. The method of claim 9, wherein themethod is performed in a temperature ranging from about 200 to 500° C.29. The method of claim 5, wherein the initial tungsten film is formedby supplying WF₆ gas as the tungsten-containing gas and SiH₄ gas as thereduction gas, and wherein the main tungsten film is formed on theinitial tungsten film by supplying WF₆ gas as the tungsten-containinggas and H₂ gas as the reduction gas.
 30. The method of claim 1, wherein,in the tungsten film forming step, the tungsten-containing gas is WF₆gas and the reduction gas includes SiH₄ gas.
 31. The method of claim 16,wherein, in the tungsten film forming step, the tungsten-containing gasis WF₆ gas and the reduction gas includes SiH₄ gas.
 32. The method ofclaim 16, wherein a TiN film is formed on the surface of the object, andwherein the tungsten film is formed on the TiN film.